One of the reasons for my suggested replacement is to use terminology more consistent with the world outside of RC.SE. I offer as Wikipedia as evidence that "mechanical-computing" is more consistent with the rest of the world than "mechanical."
No - it is redundant and serves no purpose.
RetroComputing is, as the name states, already about computing. That's the all covering base. So any tag is by default meant as a subset of computing. Thus there is not only no need to add it explicite a second time, but doing so defies structure and logic.